A water-absorbent resin (SAP/Super Absorbent Polymer) is a water-swellable and water-insoluble polymeric gellant, which is widely and mainly used in disposable articles including an absorbent article such as a disposable diaper, a sanitary napkin, and the like; an agricultural water retaining agent; and an industrial waterproofing agent. As a material for the water-absorbent resin, varieties of monomers and hydrophilic polymers have been proposed. Among them, a polyacrylic acid (salt)-based water-absorbent resin, which is formed by using the acrylic acid and/or its salt as a monomer, is most widely used in industrial purposes due to its high water absorption performance.
As a basic physical property of the water-absorbent resin, a high water absorption coefficient is desired and such a high water absorption coefficient can be defined by a measurement method such as centrifuge retention capacity (CRC/Centrifuge Retention Capacity), free swell capacity (FSC/Free Swell Capacity), etc. As a process for producing the water-absorbent resin, a large number of methods for improving the water absorbency have been proposed.
For example, Patent Literatures 1 to 4 propose methods for polymerizing un-neutralized monomers and thereafter carrying out post-neutralization (commonly known as: neutralization after acid polymerization); Patent Literature 5 proposes a method for controlling the levelness of belt polymerization; Patent Literatures 6 and 7 propose methods for controlling a polymerization temperature in a constant range; Patent Literatures 8 to 10 propose methods for scheming pulverization methods of gel after polymerization; Patent Literature 11 proposes a method for scheming drying conditions; etc.
Further, to improve residual monomers and soluble matter, methods for obtaining water-absorbent resins by polymerizing the acrylic acid or monomers with little impurities have been proposed and for example, known are methods using the acrylic acid with a little of protoanemonine and furfural (Patent Literatures 12 and 13); a method using the acrylic acid with 20 ppm or less of allyl alcohol and allyl acrylate (Patent Literature 14); and a method using the acrylic acid with a little of an acrylic acid dimer or oligomer (Patent Literature 15). Moreover, as a countermeasure for preventing clogging of pipes due to easy precipitation of maleic acid, which is an impurity of the acrylic acid, a method (Patent Literature 16) of using the acrylic acid with 50 ppm or less of maleic acid has been proposed. Furthermore, also known are methods (Patent Literatures 17 and 18) for suppressing β-hydroxypropionic acid in the acrylic acid salts after neutralization to 1000 ppm or lower. Even more, also known are a method (Patent Literature 19) for suppressing the total amount of acetic acid and propionic acid in the acrylic acid to 400 ppm or lower in terms of acidic smell of the water-absorbent resin, and a method (Patent Literature 20) for suppressing the total amount of hydroquinone and benzoquinone in monomers to 0.2 ppm or lower in terms of coloring.